Seven Forms of Rarity - Deborah Rabinowitz, 1981
For more of her work see
Blurb provided by Kevin Gaston
In this paper, Rabinowitz addresses the problem of the often ambiguous meaning of the word “rare” in terms of species distributions, and provides commentary on how natural selection operates on rare species.
Rabinowitz ascribes three variables that may contribute to the designation of rare: geographic range, habitat specificity, and dominance. When a species meets the criteria of large range with wide habitat specificity and dominance (red), it is not considered a rare form. The other seven permutations of these conditions are the titular forms deemed rare in some form or fashion.
Rabinowitz applies this theory in context of North American flora. She provides examples of each form of rarity (Figure 2, below). The glaring exceptions are plants with a small range, wide specificity (generalists), and non-dominance of which there appear to be no readily identifiable examples.
She notes that the combination of small geographic range with wide habitat specificity seem to be the rarest combinations. She suggests that one reason that may cause this phenomenon is that small populations are susceptible to demographic stochasticity, a process analogous to genetic drift. If these small populations are lost, the remaining population could be considered an endemic.
An interesting point presented in this paper is that sparse species are superior competitors to common grasses, but they fail to capitalize on this competitive advantage in terms of population density. Sparse species grow largest when densities are low. This is presumed to be an effect of stronger interspecific competition crippling an individual’s ability to grow. “Sparse species grow best when sparse, and common species grow best when common.” Despite the competitive advantage provided when rare, natural selection cannot select for rarity in a population. Individuals may prosper when rare, but this advantage may be a mechanism to offset the disadvantage of rarity.
With a practical application of Rabinowitz’s theory, the variable of seasonal or temporal shifts in population density comes into play as well. Many plants (particularly annuals) alter densities depending on yearly precipitation and seasonality. A striking example occurs in newts, which congregate in large numbers in ponds during mating season and disperse to terrestrial habitats following the bonanza.
Continued Work:
Rabinowitz’s work has been highly influential in generating follow-up studies, and I’d like to highlight a similar analysis performed in mammals (Yu and Dobson, 2000). This study provides a quantitative approach largely lacking in Rabinowitz’s treatment. Yu and Dobson examined 1212 species of mammals and evaluated which form of rarity each species should fall in given its value in each variable relative to the median. While this is certainly a paper worth reading, I’ll use it only as it directly relates to Rabinowitz. Figure 1 is included to show the alphabetic designations used for each category in Figure 2.
In Figure 2 (below), we see the number of species assigned to each form of rarity (red) represented graphically. The expected results (white bars) are interesting, but the methods and results of this paper could stem a productive discussion in and of themselves, and I want to focus on the actual number of species in each category (black bars). Recall that Rabinowitz predicted categories E and F to be the rarest in the plant world, going so far as to question whether F may not exist.
That prediction is not supported in mammals, with category F as the third most frequent. This raises a host of interesting questions about whether mammals differ from plants due to aspects of life histories or if a similarly quantitative analysis of plants would reveal similar patterns.
Discussion questions:
- Are there other ways that species can be rare?
- How would you account for temporal/ seasonal shifts in density?
- What are some of the practical applications of this study in regard to policy on endangered/ threatened species?
For more of her work see
Blurb provided by Kevin Gaston
In this paper, Rabinowitz addresses the problem of the often ambiguous meaning of the word “rare” in terms of species distributions, and provides commentary on how natural selection operates on rare species.
Rabinowitz ascribes three variables that may contribute to the designation of rare: geographic range, habitat specificity, and dominance. When a species meets the criteria of large range with wide habitat specificity and dominance (red), it is not considered a rare form. The other seven permutations of these conditions are the titular forms deemed rare in some form or fashion.
She notes that the combination of small geographic range with wide habitat specificity seem to be the rarest combinations. She suggests that one reason that may cause this phenomenon is that small populations are susceptible to demographic stochasticity, a process analogous to genetic drift. If these small populations are lost, the remaining population could be considered an endemic.
An interesting point presented in this paper is that sparse species are superior competitors to common grasses, but they fail to capitalize on this competitive advantage in terms of population density. Sparse species grow largest when densities are low. This is presumed to be an effect of stronger interspecific competition crippling an individual’s ability to grow. “Sparse species grow best when sparse, and common species grow best when common.” Despite the competitive advantage provided when rare, natural selection cannot select for rarity in a population. Individuals may prosper when rare, but this advantage may be a mechanism to offset the disadvantage of rarity.
With a practical application of Rabinowitz’s theory, the variable of seasonal or temporal shifts in population density comes into play as well. Many plants (particularly annuals) alter densities depending on yearly precipitation and seasonality. A striking example occurs in newts, which congregate in large numbers in ponds during mating season and disperse to terrestrial habitats following the bonanza.
Continued Work:
Rabinowitz’s work has been highly influential in generating follow-up studies, and I’d like to highlight a similar analysis performed in mammals (Yu and Dobson, 2000). This study provides a quantitative approach largely lacking in Rabinowitz’s treatment. Yu and Dobson examined 1212 species of mammals and evaluated which form of rarity each species should fall in given its value in each variable relative to the median. While this is certainly a paper worth reading, I’ll use it only as it directly relates to Rabinowitz. Figure 1 is included to show the alphabetic designations used for each category in Figure 2.
In Figure 2 (below), we see the number of species assigned to each form of rarity (red) represented graphically. The expected results (white bars) are interesting, but the methods and results of this paper could stem a productive discussion in and of themselves, and I want to focus on the actual number of species in each category (black bars). Recall that Rabinowitz predicted categories E and F to be the rarest in the plant world, going so far as to question whether F may not exist.
Discussion questions:
- Are there other ways that species can be rare?
- How would you account for temporal/ seasonal shifts in density?
- What are some of the practical applications of this study in regard to policy on endangered/ threatened species?
Comments
A way to account for seasonality, particularly in consumers, might be to model abundance of resources across the landscape and add that in as a factor contributing to rarity. It would make things more complicated, though, especially because quantifying resource availability and e.g. knowing the complete diet of a consumer would be challenging.
I think that using these seven categorizations of rarity (and applying their relative rankings like Yu and Dobson) would be a useful way to set up priorities for conservation of threatened species.
I think another way that species can be rare may depend on human perception. While we usually understand rarity in the way that Rabinowitz defines it in terms of range, habitat specificity, and population size, other species may be considered rare that don't necessarily fit into those categories. For example, a species with secretive/mysterious behavior or inaccessible habitat may be rarely encountered by humans, and thus considered rare.
- I wish Rabinowitz would have addressed the role of competition for these narrow endemics-- the ones with narrow ranges and habitat specificity. What I've found in the systems I've worked in is that there's this relationship between a related, widespread, generalist and a rare, narrow-range, specialist. The specialist is only found in particular habitats, like vernal pools or serpentine rock (and usually comes with rarity and/or endangered ranking) and what researchers have found, at leas in annual asters, is that these rare species are not necessarily preferring these habitats but being forced into them due to their inferior ability to compete. Is it even worth addressing this? Is this the assumed undertone of "habitat specialists"? Is it always the case that they are inferior competitors or are there instances where they're actually adapted for a particular environment and do poorly in other habitats even in the absence of competition? Are they rare specialists or are they just very tolerant?
- Factoring reproductive rarity: Wouldn't this already be accounted for in relative abundance and range? I think it's an interesting lens to apply once you've entered into one of these seven forms, but I don't think it's appropriate to make it another factor-- I feel like it would be redundant.
- As far as mammals bucking rare form F, compared to plants. If I understand it correctly these are rare species with narrow ranges but wide habitat specificity. If so, wouldn't the mobility of animals get at this?